docs: def context_chunk(self, context, j):
          """
          Retrieve the tokens in the ``j``th chunk of context ``context``.
  
          Parameters
          ----------
          context : str
              Context name.
          j : int
              Index of a context chunk.
  
          Returns
          -------
          chunk : list
              List of tokens in the selected chunk.
          """
  
          N_chunks = len(self.contexts[context])
          start = self.contexts[context][j]
          if j == N_chunks - 1:
              end = len(self)
          else:
              end = self.contexts[context][j+1]
          return [self[i] for i in xrange(start, end)]